The present invention relates to a golf ball material, and more particularly to a golf ball material that can be suitably used as the material forming a constituent part of the core and/or cover of a golf ball composed of a core of one or more layer and a cover of one or more layer.
Ionomer resins have hitherto been widely used as the cover material in golf balls. However, owing to the constant desire by users for golf balls endowed with a high rebound and a high durability, a variety of improvements are being carried out on such ionomer-based resins. In addition, innovations that entail mixing a low-cost resin such as polyethylene or ethylene-vinyl acetate copolymer (EVA) with an ionomer-based resin material have also been proposed in order to reduce costs.
One example is a golf ball that uses as the cover material an ionomer blend obtained by mixing a small amount of a medium- or low-density polyethylene with an ionomer-based resin (JP-A S60-145160). In addition. JP-A S60-80469, JP-A H01-32878, JP-A 1-104-126168 and JP-A H06-327794 describe, as golf ball materials, compositions obtained by blending an ethylene-vinyl acetate copolymer (EVA) with an ionomer-based resin. However, in all of these golf ball materials, the compatibility between the ionomer and these various types of polyolefins is poor, which has presented a problem in terms of the durability of the golf ball.
JP-A H11-276638 discloses that, by introducing a graft copolymer in which some or all of an ethylene copolymer has been graft-modified with an unsaturated dicarboxylic anhydride, the compatibility between the ionomer and the ethylene copolymer is improved, enhancing the durability of golf ball covers formed of such polymer compositions. However, a sufficient durability has not been achieved.
In addition, because the durability markedly declines when a low-cost resin such as polyethylene or ethylene-vinyl acetate copolymer (EVA) is mixed with an ionomer resin, art that improves the durability by adding and kneading an organic peroxide into this polymer alloy has been proposed. However, carrying out such a kneading operation takes time and effort. Moreover, in terms of the golf ball manufacturing operations, in cases where a compatibilizing agent is to be added to a polymer composition of an ionomer with a polyethylene, the polymer composition must first be subjected to a compounding step in an extruder, which takes a good deal of time and effort.
Golf balls that use an ionomer resin in the cover material have an excellent ball rebound, but fall short in terms of durability and flexibility. In order to compensate for this shortcoming, JP-A S56-83367 and JP-A S62-275480 disclose polymer blends of a polyester block copolymer that is flexible and has excellent resilience mixed together with an ionomer resin that has excellent resilience. This approach takes advantage of the good qualities of both resins, and is particularly effective in achieving both a good rebound performance and a good flexibility. However, because the compatibility between the polyester block copolymer and the ionomer in this mixture is poor, when such a mixture is used as a golf ball cover material, the ball lacks a sufficient durability to repeated impact.
JP-A 2005-253962 describes a mixture of an ionomer resin with a polyester elastomer in which the above problem of compatibility is ameliorated by adding an isocyanate to the mixture. As a result, a scuff resistance improving effect is obtained, but the durability to repeated impact remains a concern. The same publication indicates that, for mixtures of ionomer resins and polyamide elastomers as well, adding an isocyanate improves the compatibility and has a scuff resistance enhancing effect. However, here too, the durability to repeated impact remains a concern.
A variety of ionomer materials endowed with a high resilience and a high hardness have been proposed in order to obtain golf balls of excellent feel at impact, rebound and flight properties. For example, JP-A H10-127822, JP-A H10-137363, JP-A H10-137364, JP-A H11-299933 and JP-A 2017-8201 describe modified ionomer materials which are ionomers modified by the addition of amine compounds. However, all of these have the drawback of an inadequate durability to repeated impact.